This invention relates to an in situ method for quantifying the liquefaction tendency of water saturated soils, and for determining the potential of electro-osmosis to prevent soil liquefaction.
Soil liquefaction results from an increase in soil pore water pressure induced by transient or repeated ground motions or shocks. Pore water increases may be induced by earthquakes, explosions, impacts, and ocean waves. Soil liquefaction occurs in water saturated, cohesionless soils and causes a loss of soil strength that may result in the settlement and/or failure of buildings, dams, earthworks, embankments, slopes, and pipelines. Liquefaction of sands and silts has been reported in almost all of the major earthquakes around the world. The imposed ground stress waves from earthquakes or other transient or repeated loading induces shaking or vibratory shearing of saturated loose fine sand or silts, causing a phenomenon known as liquefaction. When loose sands and silts are subjected to repeated shear strain reversals, the volume of the soil contracts and results in an immediate rise in the pore water pressure within the soil. If the pore water pressure rises sufficiently high, then the soil grain to grain contact pressure drops to zero, and the soil mass will lose all shear strength and temporarily act like a fluid, i.e. an occurrence of liquefaction occurs. Such temporary loss of shear strength can have a catastrophic effect on earthworks or structures founded on these deposits. Major landslides, settling or tilting of buildings and bridges and instability of dams or tailings ponds and failure of pipelines have all been observed in recent years and efforts have been directed to prevent or reduce such damage.
The factors that effect the occurrence of liquefaction are soil type, grain size distribution, compactness of the soil, soil permeability, magnitude and number of the shear strain reversals. Fine sand or fine cohesionless soils containing moderate amounts of silt are most susceptible to liquefaction. Uniformly graded soils are more susceptible to liquefaction than well graded soils, and fine sands tend to liquefy more easily than coarse sands or gravelly soils. Moderate amounts of silt appear to increase the liquefaction susceptibility of fine sands; however, fine sands with large amounts of silt are less susceptible although liquefaction is still possible. Recent evidence indicates that sands containing moderate amounts of clay may also be liquefiable.
In very coarse sands or gravel, ground water can flow freely enough that pore water pressures never become dangerously high to give rise to liquefaction. Fine sands and silty sands however have moderate to low permeability, which prevents the dissipation of induced pore water pressures and results in liquefaction of the soil. If the soil pore water pressures generated during an earthquake event can be relieved, then the soil will not liquefy and hence will remain stable.
Conventional soil stabilization methods to minimize or prevent liquefaction consist of one of five general methods:
1) remove liquefaction prone soil material and replace with sound material,
2) provide structural support to underlying firm soil strata, e.g. piling,
3) densify the soil to render it less susceptible to liquefaction,
4) strengthen the liquefaction prone soils,
5) provide drainage to prevent build up of soil pore water pressures, e.g. stone or gravel columns or relief wells.
The above methods have proven successful in minimizing liquefaction related damage; however, they are expensive, difficult to implement in existing structures and some of the methods are severely limited in their effectiveness in fine grain soils. An alternative method of preventing soil liquefaction involves activating an electro-osmotic gradient away from the foundation of the structure or towards a series of pressure relief wells, and thus negate the impact of the earthquake shaking on raising the soil pore water pressure and hence maintain the soil shear strength and structural stability.
Electro-osmosis involves the application of a constant d-c current between electrodes inserted in the saturated soil, that gives rise to pore fluid movement from the source electrodes towards the sink electrodes and thus modifies the soil pore water pressures. Electro-osmosis has been used in applications such as 1) improving stability of excavations, 2) decreasing pile driving resistance, 3) increasing pile strength, 4) stabilization of soils by consolidation or grouting, 5) dewatering of sludges, 6) groundwater lowering and barrier systems, 7) increasing petroleum production, 8) removing contaminants from soils, and 9) for preventing liquefaction of soils during earthquake events. Electro-osmosis uses a d-c electrical potential difference applied across the saturated soil mass by electrodes placed in an open or closed flow arrangement. The d-c potential difference sets up a constant d-c current flowing between the source and sink electrodes. In most soils the soil particles have a negative charge. In those negatively charged soils, the source electrode is the anode electrode and the sink electrode is the cathode electrode, and ground water migrates from the anode electrode toward the cathode electrode. In other soils, such as calcareous soils, the soil particles carry a positive charge. In those positively charged soils, the source electrode is the cathode electrode, the sink electrode is the anode electrode, and ground water migrates from the cathode electode toward the anode electrode.
An xe2x80x9copenxe2x80x9d flow arrangement of the electrodes allows an ingress or egress of the pore fluid. Due to the electrically induced transport of pore water fluid, the soil pore water pressures are modified to enable excavations to be stabilized or pile driving resistance to be lowered. Electro-osmosis is not used extensively due to the high cost of maintaining the d-c potential over long periods of time and the drying out and chemical reactions that occur if the system is activated for long periods of time. For short term stabilization by pore water pressure reduction, electro-osmosis is very effective in fine grained soils, such as fine sands, silty sands, and silts.
For existing or planned structures, the liquefaction tendencies of a site need to be examined and quantified so that preventive measures can be incorporated into the design of the planned structure or the existing structure be appropriately modified. Therefore, there is a need for a definitive method of measuring the liquefaction potential of a soil in situ, quantifying under what loading conditions the soil will liquefy, and also in determining if liquefaction preventative measures such as electro-osmosis are applicable.
Prior methods for evaluating the liquefaction potential of soils consist of two basic approaches, laboratory tests and in situ tests. The laboratory methods require undisturbed soil samples which are difficult to impossible to obtain. The laboratory test methods involve cyclic triaxial, cyclic direct shear, and cyclic torsional triaxial tests. All of these tests apply a cyclic shear stress reversal upon the soil specimen. At the present time, there is not a method for obtaining undisturbed samples, in which the in situ stress state, void ratio, or structure have been preserved in cohesionless soils. Therefore, laboratory methods are considered only qualitative tests in assessing the potential of a soil to liquefy. The in situ methods currently consist of five (5) types, with four (4) of the methods being indirect empirical methods and the fifth (5th) method being a direct in situ measurement of a soil""s shear strength and an inferred method for quantifying a soil""s potential to liquefy. The four (4) indirect empirical methods are; 1) the Standard Penetration Test (SPT); 2) the Cone Penetration Test (CPT); 3) the Piezocone Penetration Test (PCPT) and 4) the Seismic Waves Test (SWT). The fifth direct in situ measurement is the Piezo Vane Test (PVT).
The Standard Penetration Test (SPT) approach is based on an empirical correlation between the number of blows of a probe to penetrate the soil correlated to the observed occurrence or non-occurrence of liquefaction of particular soils during past earthquake events. The Cone Penetration Test (CPT) has several advantages over the SPT, but like the SPT test, involves correlating cone penetration resistance to the observed occurrence or non-occurrence of liquefaction of sites during past earthquake events. The Piezocone Penetration Test (PCPT) is similar to the CPT test except that pore water pressure measurements are recorded during driving of the cone into the soils, however the quantification of a soil""s potential to liquefy is based on similar empirical relationships as the CPT test, resulting in this test having the same basic disadvantages and deficiencies as the CPT method. The Seismic Waves Test (SWT) is based on empirical relationships between the seismic wave velocities of the soil as measured in situ to the observed occurrence or non-occurrence of liquefaction at sites during past earthquake events.
The Piezo Vane Test (PVT) involves the in situ failure of the soil under a shearing action by rotating a vane inserted into a borehole drilled into the soil. The method has been utilized primarily for quantifying the shear strength of cohesive soils; however, recently it has been applied to infer the potential of a soil to liquefy, by the measurement of pore water pressures during the shearing process. The method is based on shearing the soil under totally different stress states to that experienced during an earthquake, and in most cases the soils fail immediately. Therefore, the PVT does not impose shear stress reversals upon the soil that would be experienced during an actual earthquake event.
In situ pressure meter tests have been utilized for many years both for quantification of a soil""s deformational properties but also its strength properties. These devices have either been inserted into a drill hole, driven or been self boring. Self boring pressure meters minimize any disturbance to the soil and thus only minimally deform the soil prior to the test. In the case of driven probes, empirical correction factors have been applied to account for the disturbance of the soil caused during driving of the device. Pressure meter devices have not been developed that can determine the potential of a soil to liquefy.
Therefore, there is an apparent need for a method of quantifying in situ a soil""s potential to liquefy under transient and repeated loading such as experienced during an earthquake. Such a method has to cyclically place the soil in situ under stress conditions that are analogous to those generated during an earthquake, i.e. shear stress reversal. Also there is a need for a method to quantify in situ the impact of electro-osmosis in preventing soil liquefaction by reducing pore water pressures during cyclic shear stress reversals.
A method for determining the liquefaction potential of a water saturated soil is provided in which a driven or self boring probe with a plurality of expanding and contracting bladders imposes a cyclic shear stress reversal on a body of soil in situ, and the liquefaction potential of the soil can be quantified from the subsequent measurement of pore water pressure response during the cyclic shear stress reversals. A pore water pressure increase during cyclic shear stress reversals, indicates a contractive soil which has the potential to liquefy. The method can also quantify the potential of electro-osmosis in preventing soil liquefaction, by energizing electrodes by a d-c power source during the onset of liquefaction, and by measuring the reduction in pore water pressure during subsequent repeated shear stress reversals imposed on the soil by the device.
The present invention is a method for determining in situ the liquefaction potential of a water saturated soil by placing a body of soil under cyclic shear stress reversals, under zero volume change, and undrained pore water conditions and by measuring the subsequent induced pore water pressure response. A pore water pressure increase during cyclic shear stress reversals indicates a contractive soil which has the potential to liquefy. The cyclic shear stress reversal is imposed on the soil by a driven or self boring probe with a plurality of expanding and contracting bladders which impose a cyclic stress reversal on a body of soil in situ. The simultaneous expansion and contraction of the bladders under a zero volume change condition is achieved by cyclic upward and downward vertical movement of a piston inside a fluid pressure cylinder connected to the plurality of bladders. The fluid system ensures the bladders are simultaneously expanded and contracted under zero volume change. The soil stress state varies from a horizontal maximum principal stress during the expansion phase of the bladder, and changes to a vertical maximum principal stress state during the contraction phase of the bladder. Thus the soil immediately in the zone of influence of the bladders undergoes shear stress reversals, much like that imposed in a cyclic triaxial laboratory test, except that the test is conducted in situ on an undisturbed soil mass. The pore water pressure in the soil is measured during this cyclic shear stress reversal loading which has been imposed under zero volume change and undrained conditions. The magnitude of the loading, number of cyclic shear stress reversals and change in pore water pressure quantifies the potential of the soil to liquefy.
The device is either driven to depth, much like a CPT, or inserted into a drilled borehole, or self bored into the soil. A self boring device imposes the minimal disturbance on the soil and thus provides direct measurement of a soil""s potential for liquefaction. A driven device will slightly disturbed the soil during driving and generally strengthens and stiffens the soil from its undisturbed state. An empirical correlation relationship will need to be quantified to interpret the results from the driven probe to account for the slight modification of the soil""s state from its original undisturbed in situ state. During insertion of the device to the correct depth horizon, the piston in the fluid pressure cylinder is held in the neutral or equilibrium position. Once at the measurement horizon, the device is clamped at the surface to prevent movement of the device by securing the outer most connecting rods. The data acquisition is activated, the bladders are cyclically expanded and contracted, and the pore water pressure response monitored to quantify the soil""s potential to liquefy.
The piston driving the expansion and contraction of the bladders can be cyclically moved upwards and downwards either from the surface or by electro-mechanical or hydraulic means by suitable apparatus included down hole within the device. The frequency of the expansion and contraction of the bladders is conducted at similar frequencies to shear stress reversals experienced by soil during actual earthquake events, i.e. approximately 1 Hz. The pore water pressure can be measured by a number of devices, either similar to those in PCPT equipment, or by more precise devices such as a differential pressure gauge. The pore water pressure gauge can be a strain gauge device, a piezo-electric, a vibrating wire, or other device that provides an analogue output for connection and recording to a computerized data acquisition system. In addition to monitoring the pore water pressure, the data acquisition system also simultaneously records the piston motion via an instrumented Linear Variable Differential Transformer (LVDT) and the fluid pressure in each bladder by fluid pressure gauges. Thus the loading state, the frequency of loading, and the pore water pressure response can all be recorded by the computerized data acquisition system simultaneously. From analysis of these data, the potential of the soil to undergo liquefaction can be quantified.
Thus the liquefaction potential of the soil can be quantified in situ by the method and apparatus from the subsequent measurement of pore water pressure during the cyclic shear stress reversals imposed on the soil by the device under zero volume change and under undrained pore water conditions. A pore water pressure increase during cyclic shear stress reversals, indicates a contractive soil which has the potential to liquefy. The method and apparatus can also include the activation of a d-c potential difference across electrodes contained within the device to impose an electro-osmosis gradient from the soil mass, undergoing cyclic shear stress reversals, towards pore water pressure relief electrodes. The pressure relief or sink electrodes contained within the device are energized under a opposite polarity to the source or driving electrode, and the pressure relief or sink electrodes contain a pore water pressure relief inlet to allow pore water to enter the device under static or reduced hydraulic head conditions. The subsequent reduction in pore water pressure during the cyclic stress reversals by the electro-osmosis gradient is recorded electronically by the computerized data acquisition system, including the earlier recorded parameters plus the imposed d-c voltage and induced d-c current flowing through the soil across the electrodes. The pore water pressure relief inlets contained within the device can be opened by a variety of means either electro-mechanical, hydraulic, or mechanical means upon energizing the electrodes by the d-c power source. The potential of electro-osmosis to prevent liquefaction of the particular soil horizon can thus be quantified.